Blender wouldn't paint neither face sets not the mask anymore
The problem is the Armature. Modifiers prevent Hide, Mask, and Facesets in Sculpt mode. Usually, you get a warning like this:

Armature, Cloth, and Hair/Particles, and all the other physics are also Modifiers. It's probably a bug that Blender doesn't warn you if you have one of these 3 activated. (Dynamic Paint gives you a warning.)
To see your Facesets and Mask in Sculpt Mode again you just have to disable the Armature modifier in viewport render by clicking the little icon that looks like a monitor.

Make sure your Faceset overlays are enabled. It's not enabled in the uploaded blend file ;-)